If a chemical reaction is reversed, what happens to the enthalpy of the reaction? the enthalpy of the reaction becomes zero. the enthalpy of the reaction doubles. the sign of the enthalpy of the reaction reverses. the enthalpy of the reaction is halved

The third statement is the answer.
Enthalpy change refers to the difference in energy that the products and reactants contain. When the reaction is reversed,the only difference is the product and reactant which is now switched .Other than that,the value of enthalpy change remains the same.But since the reaction is reversed, we put a negative sign( -∆H joules) to signify that.

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The correct answer is b